Deodorized edible oils are commonly prepared by subjecting crude oils initially to chemical or physical refining and then to deodorization. Generally, chemical refining comprises degumming, neutralizing dewaxing, washing and filtrating. Physical refining, on the other hand, comprises degumming, decoloring and filtering. As these refining techniques leave different amounts of impurities in the crude oils, they may affect the subsequent deodorization conditions chosen. When, for example, the amount of impurities remaining in the oil to be deodorized is substantial due to using the physical refining technique, severe deodorization operating conditions may be necessary to obtain edible oils having the desired quality.
Deodorization basically involves the removal of, among other things, impurities that impart disagreeable odor and taste from crude oils. The impurities removed usually include free fatty acids; disagreeable odor and taste causing compounds, such as aldehydes, ketones, alcohols and hydrocarbons; and compounds formed by the heat decomposition of peroxides and pigments. These impurities should be sufficiently removed to impart the desired properties to the edible oils. For example, the fatty acids in the edible oils should be substantially reduced, to about 0.1 to 0.2%, to obtain the edible oil having the desired properties.
The deodorization of the crude oils is normally accomplished at a high temperature condition under high vacuum, in the presence of an inert stripping gas, such as steam or nitrogen. The high temperature condition promotes thermal decomposition of the undesired impurities as the inert stripping gas strips off the undesired impurities from the crude oils. The high vacuum prevents oxidative degradation of the crude oils during the deodorization.
Once the deodorization is completed, the resulting, hot deodorized edible oils are cooled. At a high temperature, the deodorized edible oils are known to be highly susceptible to oxidative degradation. Thus, by cooling, the susceptibility of deodorized edible oils to oxidation is minimized.
The cooling is usually carried out in a heat recovery unit when the hot, deodorized edible oils are provided under vacuum. The heat recovery unit, which may be or may not be an integral part of the deodorizer, is operated to recover the heat from the discharging, hot deodorized edible oils through using the incoming crude oils to be deodorized as an indirect cooling means. By utilizing the crude oils to recover heat, both the hot deodorized edible oils and the crude oils can simultaneously reach the desired temperatures which are suitable for storage and deodorization, respectively. Using a conventional tube and shell heat exchanger for the above purpose, however, is highly undesirable because an external mechanical pump, which is usually needed to overcome a pressure drop in the heat exchanger, may cause air leakage, thus oxidizing the hot deodorized edible oils under vacuum.
To improve the transfer of heat from the hot, deodorized edible oils to the crude oils, the use of mechanical agitators to agitate the hot deodorized edible oils in the heat recovery unit has been proposed. Agitation with the mechanical agitators could cause turbulent mixing of the hot deodorized edible oils, which in turn reduces the thermal boundary layer next to the cooling coil having the crude oils, thus increasing the heat transfer rate. However, the mechanical agitators do not impart sufficient agitation to cause the crude oils and hot deodorized edible oils to reach equilibrium temperature unless the heat recovery unit and its associated equipments can be made significantly large. Moreover, the mechanical agitators are difficult to employ in the heat recovery unit which needs to be sealed sufficiently to prevent oxidation of the hot, deodorized oils therein. Any leakage of air into the internal chamber of the heat recovery unit, as a result of employing the mechanical agitators, can cause severe oxidative degradation of the hot, deodorized edible oils.
To alleviate the problems associated with the mechanical agitators, steam, which is readily available in a deodorization plant, has been used to agitate or turbulently mix the hot, deodorized oils in the heat recovery unit. Injecting steam into the hot, deodorized edible oils, not only causes agitation sufficient to maximize the heat transfer rate, but also produces a positive pressure inside the heat recovery unit, thus reducing the potential for air leakage.
In spite of these advantages, the use of steam in the heat recovery unit is found to be difficult. Cooling the hot, deodorized edible oils, for instance, below 100.degree. C. is undesirable because steam condenses out as water. The presence of water in the cooled deodorized edible oils may promote bacterial growth and may cause, among other things, a cloudy emulsion or the formation of an immiscible layer. Hence, hot deodorized edible oils are usually cooled to above 100.degree. C. in the heat recovery unit and then further cooled to less than 100.degree. C. in a cooler. However, such a cooling technique is uneconomical because it increases the equipment and operating cost, without maximizing the heat recovery.
Also, steam deodorized edible oils cooled in the above manner are found to have higher tendencies to absorb air, i.e., highly susceptible to oxidative degradation.
It is, therefore, desirable to find ways to enhance the stability of the deodorized edible oils, i.e., less susceptible to oxidative degradation, and enhance the heat recovery from the hot, deodorized edible oils.